This application claims the priority of German patent document 19639324.8, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a pipe clamp with a headed bolt for securing a pipe connection, in particular pipes of the exhaust system of a motor vehicle which are to be connected to one another. More particularly, the invention relates to a pipe connection in which the end of a first pipe is pushed into an axially slit end of a second pipe and, when the headed bolt is tightened, the pipe clamp, fitted on the slit pipe end, connects the two pipe ends to one another in a frictionally locking manner.
In the case of such a pipe clamp, load is introduced not uniformly but via a plurality of contact points running on the lateral surface of the adjacent pipes. In addition, there is a local pressure point in the region of the lock of the pipe clamp. Local concentrations of stressing arise at each contact point. In unfavorable conditions, the production of the connection causes the inner pipe to expand permanently even at room temperature. In contrast, since the outer pipe, which encloses the inner pipe, is slit axially, there are no unacceptable concentrations of stressing there.
As the temperature increases, the yield point of the pipe materials decreases sharply. This means that, in the case of pipe clamps which are subjected to high stressing, the pipe material may creep, this applying, in particular, to the non-slit, inner pipe. This means that, after cooling, the inner pipe, which is clamped by the outer pipe, may be deformed to a smaller diameter. As a result, the clamping force in the connection can decrease to such an extent that the pipes which are inserted into one another can be displaced or can turn and, in addition, are no longer sealed. This may be the case, in particular, with exhaust pipes with a connection of the type generally described above, since temperatures of up to 800.degree. C. can prevail at these pipes. Stressing which exceeds the flow limit of the material of the inner pipe of connection of the type generally described above may occur, in particular, when, in the event of the exhaust system heating up quickly, there is a difference in temperature between the inner pipe, which has hot exhaust gases flowing through it, and the pipe clamp, which is cooled by the relative wind. Such a difference in temperature may be, for example, 180.degree. C. In this case, the pipe clamp exerts an increased clamping force as a result of the different heat expansions. This results, in turn, in increased permanent deformation of the inner pipe. The problems outlined above arise, for example, in the case of a pipe connection according to European patent document EP 0 222 784 B1, said pipe connection being secured by a pipe clamp of the type generally described above.
A goal of the present invention is to remedy the aforementioned problems such that it is not possible, in the case of a pipe-clamp pipe connection of the type generally described above, for there to be any reduction in clamping force which adversely affects the secureness of the pipe connection.
This and other goals have been achieved according to the present invention by providing a pipe clamp with a headed bolt for securing pipes of an exhaust system of a motor vehicle which are to be connected to one another by fitting an end of a first of said pipes into an axially slit end of a second of said pipes, said pipe clamp being fitted over the axially slit end with said headed bolt being tightened to connect the two pipe ends to one another in a frictionally locking manner, wherein the pipe clamp consists of a material which has a higher coefficient of expansion than a material of the pipes which are to be connected to one another.
This and other goals have been achieved according to the present invention by providing a pipe connection in an exhaust system of a motor vehicle, comprising: a first exhaust pipe; a second exhaust pipe having an axially slit end to be fitted over an end of said first exhaust pipe; and a pipe clamp to be fitted over said axially slit end of the second exhaust pipe, said pipe clamp being adjustable via a bolt, said bolt being rotatable to adjust an inner periphery of said pipe clamp to frictionally engage said axially split end of the second exhaust pipe and to bias said axially split end of the second exhaust pipe into frictional engagement with said first exhaust pipe, said pipe clamp being formed of a material which has a higher coefficient of thermal expansion than a material of said first and second exhaust pipes.
This and other goals have been achieved according to the present invention by providing a method of forming a pipe connection in an exhaust system of a motor vehicle, said method comprising the steps of: selecting a first exhaust pipe made of a first material having a first coefficient of thermal expansion; selecting a second exhaust pipe with an axially split end, said second exhaust pipe being made of said first material; selecting a pipe clamp made of a second material having a second coefficient of thermal expansion which is higher than said first coefficient of thermal expansion, said pipe clamp being adjustable via a bolt; fitting said axially slit end of said second exhaust pipe over an end of said first exhaust pipe; fitting said pipe clamp over said axially slit end of the second exhaust pipe; rotating said bolt to adjust an inner periphery of said pipe clamp to frictionally engage said axially split end of the second exhaust pipe and to bias said axially split end of the second exhaust pipe into frictional engagement with said first exhaust pipe.
In a state in which the pipe clamp is at a considerably lower temperature than the pipes which it is to clamp with respect to one another, the material pairing of the present invention avoids a constriction, as a result of the difference in temperature, to the extent where the inner pipe is deformed permanently due to too high a clamping force.
According to the preferred embodiment of the present invention, the headed bolt has an elongate head which, in the state in which it is mounted on the pipe clamp, is aligned parallel to the axis of the pipe clamp. The invention relates to, for example, a pipe clamp which is a metal strip which is bent in the form of a circular ring and, for the purpose of receiving a headed bolt, has ends which are designed as double-layered perforated lugs. The free end of the shank of the headed bolt is provided with a thread and serves for receiving a nut. When the pipe clamp is in the mounted state, it has a shaped clamping piece, which is located between the lugs and through which the headed bolt passes, and a shaped part between the nut of the headed bolt and the adjacent lug of the pipe clamp. Such a pipe clamp is illustrated and described, for example, in European patent document EP 0 222 784 B1, mentioned above.
The elongate bolt-head form, aligned in the axial direction of the pipe clamp, is used in order to avoid local stressing when the nut is tightened onto the headed bolt. For supporting the torque which occurs when the nut is tightened, it is important to have, for the occurrence of relatively low supporting force, a long lever arm between the force transmission which takes place at the bolt head with respect to the lug of the pipe clamp and the axis of the headed bolt. If there are low supporting forces during the transmission from the bolt head to the pipe clamp, the risk of impermissibly high local stressing at the pipe clamp decreases.
Recesses in the bolt head at points where, in particular, there is no lateral abutment against the pipe clamp are advantageous and, in some cases, even necessary if hammer-head bolts are used. This is because, if there is no recess, a relatively large volume has to be deformed, and this can result in deformation cracks in the region of the bolt heads.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.